Use of the polarization inversion phenomenon, in which the polarization of a ferroelectric is forcibly inverted, forms a periodically poled region (a poled structure) in the ferroelectric. The poled region thus formed are employed in optical frequency modulators that use surface acoustic wave, and optical wavelength conversion devices that use polarization inversion of a nonlinear polarization. This is disclosed in Patent Document 1.
FIG. 1 is a perspective view of a wavelength conversion device disclosed in Patent Document 1 (in which four devices are collectively formed). FIG. 2 is a partial enlarged perspective view of the wavelength conversion device. FIGS. 1 and 2 disclose comb-shaped electrodes 8A and 8B formed on an upper insulating layer 7. The voltage applying electrode 8A includes a rectangular-shaped main body 8A1 and a plurality of branch portions 8A2. The branch portion 8A2 extends from the main body 8A1 in the direction −X. The voltage applying electrode 8B includes a plurality of branch portions 8B2 that extend from the main body 8B1 in the direction +X. The branch portions 8A2 and 8B2 are mutually engaged. The branch portions 8A2 and 8B2 are aligned alternately in the direction along the Z axis. The branch portions 8B2 alone includes a plurality of narrow branch portions 8B3 that extend approximately in the direction −Z (a direction perpendicular to the X axis and along the surface of the substrate) on the surface of the substrate.
Applying voltages V1 and V2 performs the polarization inversion. In view of this, the voltage V1 is set at a DC voltage of 500 V, while the voltage V2 is set at a pulse voltage from 200 to 800 V. Voltages required for the polarization inversion vary depending on an offset angle θ. In the case where the offset angle θ is equal to 5 degrees, the voltages V2 and V1 are both equal to 500 V. After the polarization inversion is carried out, an intermediate of the wavelength conversion device is diced (chip processing). The dicing lines are set between the main bodies 8A1 and 8B1 and vertical to the X axis.
In order to reduce optical absorption loss caused by an adhesive layer 4, a lower insulating layer 5 is disposed on the lower surface a ferroelectric single crystal substrate 6. The lower insulating layer 5 is made of SiO2 as an under cladding layer that defines a waveguide. The lower insulating film 5 has a refractive index equal to or less than 90% of a refractive index of the ferroelectric single crystal substrate 6. The lower insulating film 5 has a thickness D5 of 0.5 to 1.0 μm. In this example, the lower insulating film 5 of SiO2 is preliminarily formed over a bonding surface of the ferroelectric single crystal substrate 6. The lower insulating film 5 is laminated over a base substrate 2 via the adhesive layer 4.
As electrodes used for the polarization inversion, a metal film 3 is preliminarily formed over the surface to be bonded of the base substrate 2. Material of the metal film 3 is preferably Ta, Al, Ti, Au/Cr, or the like in view of bonding strength with the base substrate 2 and stabilization. The material may be, for example, Au(200 nm)/Cr(50 nm).
In order to decrease in deformation of the base substrate 2 when bonded to the ferroelectric single crystal substrate 6 as much as possible, a difference of thermal expansion coefficients between the base substrate 2 and the ferroelectric single crystal substrate 6 is equal to or less than 5%. That is, the ferroelectric single crystal substrate 6 has a thermal expansion coefficient in each direction of the horizontal surface of a value within the range of 95 to 105% of a thermal expansion coefficient of the base substrate 2 in each direction of the horizontal surface. These thermal expansion coefficients approximately match each other. This reduces substrate delamination and transmission loss increase caused by the difference between the thermal expansion coefficients. A material constituting the ferroelectric single crystal substrate 6 is preferably a single crystal of magnesium-oxide-doped lithium niobate. The substrate is known to have high resistance to optical damage. Therefore, the wavelength of light with high intensity can be converted.
Specifically, the base substrate 2 made of a non-doped LN substrate has a thickness D2 of 0.5 mm. This thickness D2 is preferably equal to or more than 0.1 mm. The base substrate 2 has a parallelism (steps on a surface) of 0.2 μm. This parallelism is preferably equal to or less than 0.3 μm. The MgO-doped ferroelectric single crystal substrate 6 has also a thickness D6 of 0.5 mm. This thickness D6 is preferably equal to or more than 0.1 mm. The MgO-doped ferroelectric single crystal substrate 6 has a parallelism of 0.2 μm. This parallelism is preferably equal to or less than 0.3 μm. To ensure strength of the device and flatness of the device at polishing, it is further preferable that the thicknesses D2 and D6 are both equal to or more than 0.2 mm.
The base substrate 2 and the ferroelectric single crystal substrate 6 have the identical crystal orientation.
In order to reduce the optical absorption loss caused by the adhesive layer 4, an upper insulating film 7 is disposed on the top surface of the ferroelectric single crystal substrate 6. The upper insulating film 7 is made of SiO2 as an over coating layer that constitutes an upper cladding layer of the waveguide. The upper insulating film 7 has a refractive index equal to or less than 90% of the refractive index of the ferroelectric single crystal substrate 6. The upper insulating film 7 has a thickness D7 of 0.2 to 0.5 μm.
A spacing (a cycle) X2 between centers of the narrow branch portions 8B3 of the electrode formed on the upper insulating film 7 is equal to a spacing of 6.62 μm between centers of poled regions PR in the direction X. A width X1 of the narrow branch portion 8B3 of the electrode formed on the upper insulating film 7 is equal to a width of 0.5 μm of the poled region PR in the direction X. In this case, this device functions as an SHG device of infrared laser light with a wavelength of 1.064 μm. Offset distance W3 in Z-direction between the branch portions 8A2 and 8B2 of the electrode on the substrate surface is set to 150 μm. These electrodes are fabricated by metal sputtering and subsequent photolithography. The voltage applying electrodes 8A and 8B employ a material of, for example, Au(200 nm)/Cr(50 nm).
The following describes a voltage applying method for the inverting polarization. Spontaneous polarization of the ferroelectric single crystal substrate 6 is aligned in the Z-axis direction of the crystal. Thus a direction of the polarization inversion is the opposite direction of the Z-axis direction. Therefore, the voltages V1 and V2 are applied such that the electrode 8A is at the positive side, while the electrode 8B and the metal film 3 are at the negative side. This generates an electric field EZ inside of the material between the electrode 8A and the electrode 8B, and an electric field EY inside of the material between the electrode 8A and the metal film 3. When the resultant electric field ES in the direction −Z is larger than a coercive electric field value of the ferroelectric single crystal, the polarization is inverted.
In short, a pair of the voltage applying electrodes 8A and 8B is formed on the upper insulating layer 7. The Z axis of the ferroelectric single crystal substrate 6 has an angle of θ relative to a direction of the substrate surface. The angle θ is set such that the Z axis is aligned with the direction of the electric field ES generated inside of the ferroelectric single crystal substrate 6. The electric field ES is generated by applying the voltage V1 to between the voltage applying electrodes 8A and 8B and applying the voltage V2 to between the metal film 3 and the voltage applying electrode 8A alone. Applying the voltages V1 and V2 generates the polarization inversion in the Z axis of the ferroelectric single crystal substrate 6. Thus the resultant electric field ES aligned with the Z axis reduces the voltage value required for the polarization inversion.
The above-described ferroelectric single crystal has a coercive electric field value of about 4 to 5 kV/mm. An electric field inside of the material with a lager value than the coercive electric field value is required to generate the polarization inversion. A conventional polarization inversion process applies a voltage to a bulk crystal wafer with a thickness of 0.5 to 1 mm. Therefore, the voltages V1 and V2 of a few to tens of kV have been required.
In the embodiment, the ferroelectric single crystal substrate 6 is laminated on the base substrate 2 and subsequently polished to be thin. The voltage is then applied. Therefore, the internal electric field EZ in the horizontal direction is approximately the same as before, while the internal electric field EY in the vertical direction has become equal to or more than 100 times as large as the conventional one. This increases contribution of the internal electric field EY in the vertical direction to the electric field ES in the direction of the polarization inversion. This results in small voltages in both of the directions. In the embodiment, the ferroelectric single crystal substrate 6 is polished to have a thickness D6 of 5 μm and the polarization inversion was subsequently carried out.
Applying voltages forms a periodically poled structure PPS, which is formed of a plurality of poled regions PR. Use of the wet etching or the like then forms two grooves GR1 and GR2 that extend in the X-axis direction across the plurality of the poled regions PR. This forms what is called a core of ridge-shaped waveguide. This is disclosed in Patent Document 1.
Patent Document 1: JP-A-2007-183316